WASHINGTON—The nation's most powerful gun-rights lobby called Friday for armed security guards in schools, saying that children had been left vulnerable in their classrooms.

Wayne LaPierre, executive vice president of the National Rifle Association, said that "the monsters and the predators of the world" have exploited the fact that schools are gun-free zones. Other important institutions—from banks to airports to sports stadiums—are protected with armed security, he said, but this country has left students defenseless.